Hi DavidJ, Audi,
"All things being equal, if the great pyramid was on rollers, and I had engines with enough torque to move it, I would choose to pull it because it would be so much easier to steer, and to apply the force efficiently."
Hmm, ok, suppose it were riding on a bed of air, maybe one might prefer to pull it (ymmmv). But, my point was that the amount of effort required to move the pyramid wouldn't change. Well, I have to admit that I'm not a physicist. Anyway, otoh, how does one usually move a car that has run out of gas --just for argument's sake, especially if the gas station is at the top of the hill. Again, ymmv.
"On the Punch to Knee and Needle to the Bottom of the Sea, try turning your shoulders to the left. With the punch your left shoulder need not drop at all."
I don't know if we're talking about the same movement. I'm thinking of the lowest punch in the second section. Anyway, Audi referred to my main concern about Needle: i.e., that some styles seem to "lean" (however we define it) and others don't. The shoulders weren't what I had in mind.
Audi, well, -imho- what is "correct" is often determined by circumstances. In an ideal world, we can practice on a flat, smooth surface. In the world we live in, allowances have to be made. I think the principles of tjq are broad enough to handle it. I can tell you, though, that there are those who deliberately practice on uneven ground, and even put artifical obstacles in the way. But, they also look at forms training as a means and not an end. Fwiw.
"I have seen performances of the modern 24-movement form that do not have a lean even in this postue, but I assume that this form has the least claim to be representing martial efficiency. Is a bending Needle at Sea Bottom simply an exception?"
Again, fwiw, but --as to the Beijing 24-- the reasons for the lack of lean have much to do with the application. It is the "weight" of the body's "sinking," not of the arm movement that, imv, enables the application. Clearly, no?, using the lean for power generation is somewhat a violation of principle: i.e., not "whole body." Otoh, as in Wu style, there are reasons for leaning, but --imho-- it is more important for the practitioner to understand why he is doing it, and why it is Not a violation of principle.
"Also, is there anyone who maintains a reasonably strict vertical posture during vigorous push hands?"
You know, I think Mario would be a good person to respond to this. My guess is that he would try. And, I personally think that, in general, maintaining "uprightness" is a better strategy (you know, the "light and flexible energy," the "as if suspended" idea. However, I think it is often the case that we have to move. And, just my opinion, but if it is just as possible to do taiji without leaning as it is to walk without "leaning."